This invention relates to fusible elements for use in electric circuits and more particularly to fusible elements having improved fatigue-resistance when subjected to cyclic loads.
Most prior art fusible elements or fusible ribbons for electric fuses have one or more portions thereof of reduced cross-sectional area which are known as "weak spots". Due to the fact that the weak spots in the fusible ribbons have reduced cross-sectional area, they have a higher ohmic resistance per unit length than the non-weak spot portions of the fusible ribbons. Consequently, at any given current load the heat build-up per unit length is greatest at the weak spot, and the weak spot is the first portion of the fusible ribbon to fuse during a severe overcurrent.
Although weak spots in fusible ribbons take many forms in the prior art, it is common for a weak spot or spots to be linearly arranged transversely to the axis of the fuse ribbon. Since a weak spot has a reduced cross-section compared with the remainder of the fusible ribbon, they represent, mechanically, the weakest sections of the fusible ribbon. Under conditions of cyclic loading, e.g. frequent on-off cycles of current flow, the fuse element or ribbon heats and cools cyclically causing mechanical stress therein due to cyclical expansion and contraction. A weak spot, having the least mechanical strength, is flexed and stressed more than the remainder of the element. The largest heat build-up per unit length of the fusible ribbon also occurs at weak spots causing stress therein due to this expansion. This leads to fatigue in the metal at weak spots and frequently will ultimately lead to fuse failure due to mechanical failure of a weak spot.